Recent developments in the wireless telephone communications industry have created the need for wireless subscriber terminals (or "wireless telephones") capable of operating in two widely displaced frequency ranges. In the United States, the frequency range from approximately 824 to 894 MHz (with some gaps) has been allocated for conventional "cellular" radio telephone service, and the frequency range from approximately 1850 to 1990 MHz has been allocated for a new "Personal Communications System" (PCS) service. Cellular systems, some of which have been in commercial operation since 1984, are relatively mature. Cellular systems provide "blanket" coverage throughout many metropolitan areas and geographically extensive coverage in many other areas where the population density or vehicular traffic are sufficient to warrant coverage.
PCS systems, on the other hand, are relatively new, and have a relatively small subscriber base. Some metropolitan areas do not yet have working PCS systems, and even in areas in which one or more PCS systems exist, such systems do not yet provide coverage which is as geographically extensive as that provided by mature cellular systems. As a result, a subscriber to a particular PCS system may often be in a location in which the subscriber's PCS system is not available, but a cooperative cellular system is available. This could occur, for example, when the subscriber is located within a coverage void in a "home" region generally served by the subscribed PCS system. This could also occur when the subscriber is located outside the home region, such as in a city where the subscriber's wireless service provider does not operate a PCS system.
In order to enable PCS system subscribers to obtain wireless telephone service in areas in which the subscribed PCS system is unavailable, but a cellular system is available, wireless telephone manufacturers have developed wireless telephones capable of operation in both the cellular and PCS frequency bands. For convenient reference, the term "cellular" as applied to frequencies or frequency bands is used herein to refer to the frequency bands allocated in the United States to the Domestic Public Cellular Telecommunications Radio Service (generally, 824 to 894 MHz), and to nearby frequencies, without regard to the type of service, radio protocol standards, or technology actually in use at such frequencies. The term "PCS" as applied to frequencies or frequency bands is used herein to refer to the frequency bands allocated in the United States to Broadband Personal Communications Services (generally, 1850 to 1990 MHz), and to nearby frequencies, without regard to the type of service, radio protocol standards, or technology actually in use at such frequencies.
Hand-held wireless telephones are typically equipped with a small, flexible antenna capable of operating, to some extent, in both the cellular and PCS frequency bands. Antennas of this type are very short, compared to the wavelength of the signals to be transmitted and received, and are therefore very inefficient. Such antennas may be adequate when the wireless telephone is used in a location which affords a relatively short, unobstructed RF path to the base station with which communication is desired. However, when the wireless telephone is used in other locations, a better antenna is needed.
In particular, when the wireless telephone is used inside a vehicle, the structure of the vehicle both obstructs the RF path between the telephone and the base station, and scatters a substantial amount of the RF energy which would otherwise be transmitted or received by the wireless telephone. Accordingly, it is highly desirable to connect the portable telephone to an efficient antenna located on the exterior of the vehicle. This is especially important when operating in the PCS frequency band. Radio signal propagation characteristics at PCS frequencies are significantly poorer than at cellular frequencies, and the transmitter power allowed at PCS frequencies is significantly lower than the transmitter power allowed at cellular frequencies.
A popular type of antenna used in cellular and other vehicular applications is a glass-mounted or window-mounted antenna. Such antennas generally include an external portion semi-permanently affixed to the exterior surface of a vehicle window, and an internal portion semi-permanently affixed to an interior surface of the vehicle window at a position opposite the exterior portion. The interior portion is electrically connected to a suitable transmission line cable which, in turn, may be connected to the mobile telephone transceiver. The internal portion is electrically coupled to the external portion through the glass separating the two portions. The interior portion may incorporate a circuit for matching the impedance of the antenna to the impedance of the transmission line cable and for controlling the impedance of the coupling through the glass. In addition, the interior portion (or an element thereof) may function as a counterpoise.
Glass-mounted antennas are preferred in many applications because installing the antenna does not require drilling holes in an exterior vehicle surface for use in mounting the antenna and for passing a transmission line cable. This avoids problems with leakage of air and water into the vehicle, and allows the antenna to be removed from the vehicle without sealing or repairing the holes. Although temporarily installed antennas are available, they are visually obtrusive and require the transmission line cable to be passed through an existing door or window opening. As a result, the transmission line cables are often damaged.
A glass-mounted antenna generally as described above, for use at frequencies below those used in cellular and PCS communications, is disclosed in Parfitt U.S. Pat. No. 4,238,799, which is assigned to the assignee of the present application. Glass-mounted antennas for use at cellular frequencies are disclosed in Hadzoglou U.S. Pat. No. 4,839,660, which is assigned to the assignee of the present application, and in Larsen U.S. Pat. No. 4,764,773. It is believed that in each of these antennas, the mechanism by which coupling is achieved through the glass is primarily capacitive. Each of these antennas is designed to operate over a reasonably wide, but nonetheless limited, range of frequencies surrounding an optimum operating frequency. For example, the cellular antennas are disclosed as covering the entire U.S. cellular frequency band.
However, none of the antennas described in the aforementioned patents are designed specifically for operation in the PCS frequency band (1850-1900 MHz). Many existing cellular through-the-glass antennas tend to perform poorly in the PCS band due to reasons such as mismatched impedances, poor coupling through the glass, and distorted radiation characteristics. Similarly, many existing PCS antennas tend to perform poorly in the cellular band due to reasons such as mismatched impedances, poor coupling through the glass, and reduced antenna aperture.
Although there exist well-known techniques for modifying an existing antenna design to operate at a different frequency, such techniques often cannot be applied when the target operating frequency differs widely from the original operating frequency, because structures and materials may behave electrically in a fundamentally different manner. Moreover, even if the aforementioned antenna designs could be modified to operate at PCS frequencies, the bandwidths of the antennas are not sufficiently wide to allow them to be simultaneously adapted to operate satisfactorily at both cellular and PCS frequencies. Thus, a wireless subscriber using a "dual-band" wireless telephone in a vehicular application would be required to install two separate antennas on the vehicle.
Dual-band glass-mounted antennas for use in the 144-148 MHz and 440-450 MHz amateur radio bands have been mentioned in the sales literature of Tandy Corporation of Fort Worth, Tex. (e.g. Radio Shack part number 190-0324), and Larsen Electronics, Inc. of Vancouver, Wash. (e.g. Larsen model number KG 2/70). However, these antennas, and the structures they employ for coupling through the glass and for matching the antenna to the radio transceiver transmission line cable, are not suitable for use in the cellular and PCS frequency bands.
In addition, it is believed that these VHF/UHF antenna designs may exploit the serendipitous fact that the higher target operating frequency is almost exactly three times the lower target operating frequency. These antennas generally employ a radiator having upper and lower straight sections separated by a coiled section. The lengths of the straight sections and the parameters of the coiled section are selected such that the total radiator length is equivalent to a half wavelength at VHF. Because of the three-to-one ratio of frequencies, the developed length of the radiator consists of three half-wave sections at UHF. At VHF frequencies, the coil acts as a loading section, with the total radiator acting as a half-wavelength, unity-gain antenna. At UHF frequencies, the coil acts as a phasing element, creating a two element collinear radiator. Thus, this simple configuration works well for the 150 and 450 MHz bands because of the three-to-one ratio of frequencies.
This approach to constructing a dual-band antenna cannot be used successfully for the CELLULAR and PCS bands because the ratio of the frequency bands is on the order of two-to-one. The two-to-one frequency ratio tends to transform the low impedances to high impedances, and conversely high impedances to low impedances, between the two bands. This factor complicates the design of a dual-band antenna because it is generally desirable that the antenna present a consistent impedance, approximately matched to the transceiver with which it is to be used, at all operating frequencies.
Moreover, existing glass-mounted VHF/UHF dual band antennas employ through-the-glass couplers and associated matching circuitry which are designed to function only with a radiator exhibiting similar base impedances in both frequency bands. Thus, even if the wireless telephone transceiver could tolerate the widely disparate base impedances exhibited by prior-art radiators when used on frequency bands having a two-to-one ratio, these radiators could not be used with prior art through-the-glass couplers.